Conventionally, while communication between vehicles coupled each other by an electrical coupler is performed through contact portions in the electrical coupler, sometimes an oxide film is formed on a surface of the contact portions. In this state, a failure of contact can occur between the contact portions, causing a communication failure between the vehicles.
In this connection, Patent Literature 1 discloses a technique in which, in a signal transfer system that transfers a signal through contact portions in a connector of a wired line, an oxide film is broken by applying a direct-current component by a direct-current voltage supply applied to the contact portions. A transmission portion in this case is configured to include a transmission amplifier, a transformer connected to the transmission amplifier, a coupling capacitor connected to the transformer and the connector, and the direct-current voltage supply connected between the coupling capacitor and the connector via a choke coil.